310
BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
nondimensional solutions, and the stiffness of the upper portion of the pile is
selected. Expérience has shown that the behavior of the upper portion of a pile
under latéral loading has a significant effect on the resuit of a solution.
Solutions Using Nondimensional Parameters
The p-y curves are nonlinear, which leads to a nonlinear solution with respect
to the latéral load
therefore, a sériés of solutions should be generated for
a range of values of Pt. An approach that is useful is to increase the Pt by
multiplying the service load by the factor of safety and to check the behavior
of the pile under the increased load. However, it is wise to employ loads below
and above the design load because nature of the p-y curves is such that in some
cases a slight increase in latéral load can cause a large increase in deflection and
bending moment.
The soil conditions employed in this analysis are shown in Figure 11.14.
The soil conditions are représentative of those in the eastern Gulf of Mexico
and offshore from the Louisiana coast. The water depth is 91 ft. The undrained
shear strength and the water content vary with depth in a manner like that of
a normally Consolidated clay. The soil profile was simplified somewhat, and the
following values of undrained shear strength and submerged unit weight were
selected for the analysis:
c = 0 at x = 0; c= 12.15 lb/in.2 at x = 2400 in.
7Z = 0.020 lb/in.3 at x = 0 ;
7' = 0.036 lb/in.3 at x = 2400 in.
In the absence of stress-strain curves, the value of £50 was assumed to be 0.02.
In view of the soil conditions, the method used to compute the p-y curves
was for soft clay below the water surface, as detailed earlier. It is assumed
that the loading is cyclic because the maximum latéral load on an offshore
platform occurs during a storm. The p-y curves were computed for the following
depths in inches: 0, 50, 100, 200, 300, 400, 500, 700, and 900. In generating
nondimensional solutions, the curves are spaced more closely near the mudline.
As can be seen later, two or three additional curves between the mudline and a
depth of 300 in. would hâve been helpful.
The first step in computing the p-y curves is to compute the ultimate soil
résistance pu using the smaller of the values from équations (11-7) and (11.8).
These values are shown in Table 11.4, along with values of zr = xT computed
from équation (11.11).
Shown in Figure 11.15 are computer-generated p-y curves computed for varions depths up to 900 in., ail based on a pile diameter of 33 in. The rotational
restraint at the pile head is assumed to be constant for ail of the latéral loads.
The soil profile of Figure 11.14 shows relatively soft clay to a considérable
depth. No computations are shown here for the pile pénétration that is required
to sustain the expected axial loading; however, it is likely that the piles would
tip in the sand deposit. As noted earlier, when a pile imder latéral loading
reaches a length where it can be termed as long pile, any additional length has
BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
nondimensional solutions, and the stiffness of the upper portion of the pile is
selected. Expérience has shown that the behavior of the upper portion of a pile
under latéral loading has a significant effect on the resuit of a solution.
Solutions Using Nondimensional Parameters
The p-y curves are nonlinear, which leads to a nonlinear solution with respect
to the latéral load
therefore, a sériés of solutions should be generated for
a range of values of Pt. An approach that is useful is to increase the Pt by
multiplying the service load by the factor of safety and to check the behavior
of the pile under the increased load. However, it is wise to employ loads below
and above the design load because nature of the p-y curves is such that in some
cases a slight increase in latéral load can cause a large increase in deflection and
bending moment.
The soil conditions employed in this analysis are shown in Figure 11.14.
The soil conditions are représentative of those in the eastern Gulf of Mexico
and offshore from the Louisiana coast. The water depth is 91 ft. The undrained
shear strength and the water content vary with depth in a manner like that of
a normally Consolidated clay. The soil profile was simplified somewhat, and the
following values of undrained shear strength and submerged unit weight were
selected for the analysis:
c = 0 at x = 0; c= 12.15 lb/in.2 at x = 2400 in.
7Z = 0.020 lb/in.3 at x = 0 ;
7' = 0.036 lb/in.3 at x = 2400 in.
In the absence of stress-strain curves, the value of £50 was assumed to be 0.02.
In view of the soil conditions, the method used to compute the p-y curves
was for soft clay below the water surface, as detailed earlier. It is assumed
that the loading is cyclic because the maximum latéral load on an offshore
platform occurs during a storm. The p-y curves were computed for the following
depths in inches: 0, 50, 100, 200, 300, 400, 500, 700, and 900. In generating
nondimensional solutions, the curves are spaced more closely near the mudline.
As can be seen later, two or three additional curves between the mudline and a
depth of 300 in. would hâve been helpful.
The first step in computing the p-y curves is to compute the ultimate soil
résistance pu using the smaller of the values from équations (11-7) and (11.8).
These values are shown in Table 11.4, along with values of zr = xT computed
from équation (11.11).
Shown in Figure 11.15 are computer-generated p-y curves computed for varions depths up to 900 in., ail based on a pile diameter of 33 in. The rotational
restraint at the pile head is assumed to be constant for ail of the latéral loads.
The soil profile of Figure 11.14 shows relatively soft clay to a considérable
depth. No computations are shown here for the pile pénétration that is required
to sustain the expected axial loading; however, it is likely that the piles would
tip in the sand deposit. As noted earlier, when a pile imder latéral loading
reaches a length where it can be termed as long pile, any additional length has